OWLinearProjectioin: Issue a warning when data has no continuous variables
OWLinearProjectioin: Issue a warning when data has no continuous variables

The widget can only display continuous variables, therefore warning is shown insted of just showing empty panel.

cheral/orange3,qPCR4vir/orange3,cheral/orange3,cheral/orange3,qPCR4vir/orange3,cheral/orange3,qPCR4vir/orange3,qPCR4vir/orange3,qPCR4vir/orange3,qPCR4vir/orange3,cheral/orange3,cheral/orange3